1. Field of the Invention
The present invention relates to a semiconducting ceramic material and to an electronic part employing the same. More particularly, the invention relates to a BaTiO3 semiconducting ceramic material possessing a positive temperature coefficient of resistance and to an electronic part, such as a thermistor, employing the ceramic material.
2. Background Art
Conventionally, BaTiO3 semiconducting ceramic materials, which have a positive temperature coefficient (abbreviated as PTC) of resistance (such characteristic is referred to as a PTC characteristic), have been employed in PTC thermistors used in a wide range of applications; e.g., for demagnetization of a cathode ray tube or as an element in heaters. Furthermore, there has been a strong demand for elevating the withstand voltage of BaTiO3 semiconducting ceramic material in order to broaden its area of use, and addition of elements such as Mn and Ca to the ceramic material has been proposed.
However, sufficient withstand voltage in BaTiO3 semiconducting ceramic materials which have existing compositional proportions and which are produced through an existing method is difficult to realize, and the thickness of semiconducting ceramic sheets must be increased so as to attain the desired high withstand voltage. However, when a semiconducting ceramic sheet is incorporated into an electronic part such as a monolithic PTC thermistor, the thickness of the ceramic sheet cannot be increased beyond a certain level. Therefore, there has been a strong demand for elevating withstand voltage per unit thickness of a semiconducting ceramic material.
In view of the foregoing, the present inventors have conducted extensive studies on semiconducting BaTiO3 ceramic materials having a PTC characteristic (hereinafter referred to as xe2x80x9cPTC semiconducting BaTiO3 ceramic materialsxe2x80x9d) in terms of the relationship between withstand voltage and the temperature characteristic of resistance, and have found that controlling the temperature (hereinafter referred to as xe2x80x9cTN temperature,xe2x80x9d see FIG. 1) defined at the boundary between a first temperature range and a second temperature range to 180xc2x0 C. or more higher than the Curie temperature results in a withstand voltage remarkably higher than that of an existing similar semiconducting ceramic material, even though the resistance at room temperature is the same in both cases. The first temperature range is higher than the Curie temperature and the ceramic material has a positive temperature coefficient of resistance in this range, and the second temperature range is higher than the first temperature range and the ceramic material has a negative temperature coefficient of resistance.
PTC semiconducting BaTiO3 ceramic materials produced through a conventional method exhibit a difference between TN temperature and Curie temperature of 100-150xc2x0 C. The present inventors have elucidated that suppression of a liquid phase component to a minimum level and control of the firing temperature to a temperature at which ceramic is not completely sintered are effective measures for elevating the TN temperature and result in a remarkably high withstand voltage. The present invention has been accomplished on the basis of these findings. The expression xe2x80x9cnot completely sinteredxe2x80x9d refers to a state in which sintered ceramic grains are present in association with a volume of intergranular space. In contrast, the expression xe2x80x9ccompletely sinteredxe2x80x9d refers to a state in which sintered ceramic grains are highly densified such that substantially no intergranular space can be observed under a typical electron microscope.
Thus, an object of the present invention is to provide a PTC semiconducting BaTiO3 ceramic material having a high withstand voltage. Another object of the present invention is to provide a method of producing the semiconducting ceramic material. Yet another object of the present invention is to provide an electronic part employing the semiconducting ceramic material.
Accordingly, in one aspect of the invention, there is provided a semiconducting ceramic material comprising BaTiO3 and exhibiting a PTC characteristic, wherein the boundary temperature defined at the boundary between a first temperature range and a second temperature range is 180xc2x0 C. or more higher than Curie temperature, wherein the first temperature range is higher than Curie temperature and the ceramic material has a positive temperature coefficient of resistance in the range, and the second temperature range is higher than the first temperature range and the ceramic material has a negative temperature coefficient of resistance within this range.
Preferably, a portion of Ba atoms are substituted by Sm atoms, SiO2 is contained at a mol ratio represented by r1 of approximately 0.0005 and Mn is optionally contained at a mol ratio represented by r2 of 0 to approximately 0.0001, inclusive, the mol ratios being based on BaTiO3 serving as a predominant component.
In the present invention, the Curie temperature is an equivalent of crystal phase transition temperature in the transition from tetragonal to cubic or from cubic to tetragonal.
In another aspect of the invention, there is provided an electronic part comprising internal electrodes and a semiconducting ceramic material as recited above, the internal electrodes and the semiconducting ceramic material being alternately superposed one on another.
In yet another aspect of the invention, there is provided a method of producing a semiconducting ceramic material, comprising
mixing a BaTiO3 source, a material imparting a semiconducting property to BaTiO3, SiO2, and optionally Mn, to thereby form a mixture;
calcining the resultant mixture;
mixing the resultant calcined mixture with an organic binder;
compacting the resultant mixture, to thereby yield a compact;
firing the compact in an H2/N2 atmosphere at a temperature lower than a temperature at which the mixture is completely sintered; and
performing re-oxidation of the fired compact in air.
Preferably, the re-oxidization is carried out at approximately 1000xc2x0 C.
Preferably, the firing temperature is approximately 1225-1275xc2x0 C.
Although the temperature at which a ceramic material is completely sintered depends on the chemical composition thereof, a semiconducting ceramic material according to the present invention is completely sintered at 1350xc2x0 C.
As is the case conventionally, when the difference between the TN temperature and Curie temperature is 180xc2x0 C. or less, sufficient withstand voltage cannot be attained.